Mobile Theme - captcha
Not sure how you're going to handle things in 4.0, but currently the mobile theme log in requires no CAPTCHA if that's selected in Site Options. Makes no sense to have it for any other theme if user can select Mobile and avoid it.
Bonus tidbit, Mobile theme signup page lists the standard theme "Enter your personal information. * indicates required field." but no asterisks are shown. Edit. Hmm, and they're not displayed in the Classic theme either.